In this vein, major achievement was Neerven's analytic calculation of
the massless integrals \(I^{(j,l)}_D\)  in
Ref. [1]}. By calculating them as the discontinuity of a box graph and
employing the optical theorem he heavily influenced the approach towards PSIs.
For a long time this constituted the only published detailed calculation of angular integrals.
Other papers were heavily based on the results of Ref. [1]}.
The most commonly used reference for the angular integrals for the last three decades
is [3]}. Ref. [3]} gave the angular integral
in the form of Eq. (REF ) using the set of parameters
\(\lbrace a, b, A, B, C\rbrace \)  and produced a comprehensive, often cited list of angular integrals.
The double massive integral \(I^{(j,l)}_4\)  was given for the case \(|j|,|l|\le 2\) .
The divergent integrals were expanded up to order \(\varepsilon \) , the massless integral
was given for \(j=1,-2\le l\le 1\) , and the single massive
for \(l=1\) , \(j=-2,-1,1,2\)  and \(l=2\) , \(j=0,1,2\) . The single massive integrals were given in two groups
without making the symmetry between \(a^2\ne b^2\)  and \(A^2\ne B^2+C^2\)  explicit.
The list of the integrals was based on the calculation method put forward
in Refs. [1]}, [6]}, [7]}.
A similar list was compiled in Ref. [8]}.
Some early use of those lists can be found in Ref. [9]}.
Ref. [9]} mentioned the massless and single massive integrals \(I^{(j,l)}_D\) 
and stated that the single massive integral cannot be written in as elegant a form as
the massless integral, hence brute force methods were used.
Ref. [9]} derived an early version of two-point
partial fractioning discussed in more details in Appendix 
of the present paper. Ref. [12]} considered the angular
integration as part of the three-particle phase space. Again Eq. (REF )
appeared for the massless case. Note that only two-point partial fractioning was mentioned in
Ref. [9]} and there was
no three-point partial fractioning (see Appendix ).
So seven integrals from the list of Ref. [9]},
including those containing the massive propagator, were said to be integrated
by brute force methods. In the follow-up paper [15]}
the Neerven integral came with a reference to the 1989 paper [3]}.
The single massive integral was described as very cumbersome. About the double massive case they
write fortunately [the double massive integral] can be avoided by choosing an appropriate frame.
As a benchmark, their partial fractioning algorithm led to 217 different 3-body PSI.
